This invention is in the field of furnaces; more particularly, the invention relates to radiant tubes used to heat furnaces.
Furnaces which use radiant tube heaters as a total or partial heat source are known in the art and are in common use. Radiant tubes, as are known, have sources of air and fuel external to the furnace. The environment within the tube is independent of the atmosphere within the furnace with regard to certain properties, particularly composition and pressure. The air and fuel are mixed and burned uniformly within the radiant tube. Thereby the radiant tube provides a source of heat which radiates relatively uniformly along the length of the tube. By careful furnace design and location of such radiant tubes, a furnace can be heated to suit a particular process need.
Various furnaces in which radiant tubes are or can be used have atmospheres that contain combustible gases. Many furnace applications have high hydrogen and carbon mon-oxide low BTU or low combustion atmospheres to facilitate reducing, carburizing or similar process conditions. The atmospheres, therefore, have a significant heating value that can approach the heating value of a low BTU gas (150 BTU/cubic ft). Normally, these atmospheres are exhausted from the furnace and burned off in the environment outside of the furnace. If the energy in such a low BTU atmosphere gas could be saved, the heat savings could be substantial.
A typical example is the endothermic gas that acts as a carrier gas in a carburizing furnace. Normally, such gas is burned off outside the furnace in the environment after use. In a typical continuous carburizing furnace at least 2,000 standard cubic feet per hour (scfh) of low BTU endothermic carrier gas is used. The available heat which could be obtained from this gas could amount to over 250,000 BTU/hr. It would be desireable to provide a means to utilize the energy available in furnace atmospheres.
In the past attempts to capture the energy of the combustible gas in the furnace chamber resulted in the need for drastic structural changes in furnace design. One such example is U.S. Pat. No. 2,848,207 by F. A. Rusciano. In this patent combustible gases from the combustion chamber are vented to secondary heating chambers built into the floor of the furnace. This patent requires major structural changes in the furnace as well as a complicated venting system. The present invention allows the capturing of the energy in the combustible gas in the furnace chamber for direct use in the furnace chamber without major structural modifications in the furnace and without a cumbersome venting system.
A clearer understanding of some of the advances of the present invention over the state of the art will be noted by a review of radiant tube heaters. Of particular interest are U.S. Pat. Nos. 2,860,864; 2,764,145; and 2,873,798.
Tube materials are generally made of nickel-chrome alloys which must be able to resist the heat as well as carbon attack. Flame temperatures are higher than the temperatures which the tube can withstand for reasonable lengths of time. To overcome this problem and accommodate a general goal of uniform burning along the length of the tube, radiant burners introduce fuel or a fuel-air premix is fed into the tube along the longitudinal center of the tube. Combustion air is introduced between the fuel or fuel-air premix and the tube wall thereby providing a barrier of air between the actual flame site and the tube wall. This is of particular importance near the entrance of the tube where hot spots can be troublesome.